Stumped on cause of evident light "leak"

[Ulophot]

I am stumped by the wedge of extra density shown here. Perhaps someone has some insight. Here are the circumstances.

I took the Tachihara 4x5 out one afternoon to see what the interesting clouds and my orange 21 filter might do. I took 2 holders loaded with HP5. The camera was set up and remained stationary for all four exposures. I took my readings at the outset of the session and set the shutter speed and aperture, which also remained unchanged (1/60 at f/11 after the filter factor).

Unsure of how fast the cloud changes might occur, I pulled the slide once the holder was seated and let the camera wait, with my dark cloth folded over the back and bellows, for up to perhaps 20 minutes before releasing the shutter on a given sheet. I had no watch, so I don’t know the intervals. The camera was in direct sun. This long interval naturally comes up for suspicion.

I was facing west, a bit southerly. As you can see, the sun was high and to the left; it didn't get too frontal while I was out.

Here's the real stumper for me: the same pattern appears in the same place on all sheets, though the density varies somewhat. Since it doesn't spread into the film margin behind the holder edge, it was caused in camera (my darkroom is properly dark anyway). The density is greater at the edge than toward the center where the wedge narrows and ends. I can see no way it could come from that side; holders were seated properly. The position, in camera, was upper right (looking at the back of the camera).

Could it have come through the light trap in the holder the same way in all sheets? Unlikely, I think. The only guess I can make is some internal reflection from a surface inside the camera, but I can't figure out what it could possibly have been. The shape doesn’t look like a typical seating problem, and it's identical with both holders; nor does it look to me like any optical flare I've experienced in any format, and the sun was moving, so, were it the latter, presumably the flare position would have moved. Same for bellows, and that's one weird bellows leak, if it is one, especially with the dark cloth on top.

I can repeat the circumstances generally, perhaps that's the only way to see if it's repeatable. Meanwhile, any insights?